SIZING CRITERIA AND PERFORMANCE EVALUATION OF DIRECT AIR COOLED HEAT REJECTION UNITS FOR SUPERCRITICAL CO2 POWER PLANTS

Supercritical CO2 power plants are recognized as a promising solution for the exploitation of different energy sources: from fossil fuel combustion and nuclear energy to solar energy and waste heat recovery. The advantage of sCO2 systems with respect to ORC for small power outputs is represented by the possibility to reach higher temperatures thanks to the high thermal stability of CO2. On the contrary, for large scale power plants sCO2 technology can compete against steam cycles being likely able of fast transients thanks to the compact dimension of turbomachinery and to show outstanding part load performance. However, the reliability of sCO2 power plants is not proven yet on large scale systems and it will strongly rely on the availability of specific components. The design of the compressor is critical because of the small dimension, the high rotational speed and the need to handle a fluid close to the critical point. The recuperators design is also challenging because of the large pressure differences between cold and hot side and the need to enhance the heat transfer coefficients. The last main component, with the exception of the primary heat exchanger which depends on the specific application, is the heat rejection unit (HRU). The large range of possible applications and possible scarcity of water resources for sCO2 systems makes the use of direct air cooled HRU of great interest. This paper deals with the numerical modelization supported by experimental data of direct air cooled HRU for supercritical CO2 with pressures ranging from 70 to 100 bar and maximum temperatures between 70-150°C. Main design criteria are derived from the design of HRU of CO2 refrigeration systems that likely share many features with next generation power plants components. The effect of different heat exchangers arrangements on the overall heat transfer area, fan consumption, internal volume and footprint are discussed providing useful insights on the design and the performance of supercritical CO2 heat rejection units.